1. Field of the Invention
The present invention relates to an RF tag and a method of manufacturing the RF tag.
2. Description of the Related Art
RF tags are sometimes used to manage various objects such as commodities and articles. Such a system includes multiple RF tags and a reader/writer that reads information from or writes information onto the RF tags (hereinafter referred to as “RF tag reader”). Each object is accompanied by an RF tag. The reader is also referred to as “interrogator.” The RF tag may also be referred to as “RFID tag,” “radio tag,” or “IC tag.” For example, data such as identification information (ID), a serial number, a manufacturing date, and a manufacturing location may be written onto the RF tag.
In general, there are active RF tags and passive RF tags. Active RF tags can provide power by themselves and thus can simplify the device configuration on the RF tag reader side. The latter cannot provide power by themselves, and receive external energy to perform operations such as transmission of ID information. The passive type is preferable in terms of lowering the price of RF tags, and is particularly promising.
In terms of a signal frequency band to use, there are an electromagnetic coupling system and an electromagnetic wave system. The former uses a frequency band around several kHz, a frequency band around 13 MHz, and the like. The latter uses higher frequency bands such as the UHF band (for example, 950 MHz) and 2.45 GHz. It is preferable to use a high frequency signal in terms of enabling communications over a longer distance or reducing the dimensions of RF tags. For example, the electromagnetic coupling system is known to allow communications over no more than approximately 1 m. Further, one wavelength can be approximately 30 cm at 950 MHz but is as long as 23 m at 13 MHz.
The RF tag may accompany various objects. It is considered particularly important in designing the RF tag whether the object is conductive. If the object has an insulation quality, the operating characteristics of the RF tag do not change very greatly between before and after attachment of the RF tag. However, if the RF tag is attached to a conductor such as a metal enclosure, an image current due to the conductor is generated when the RF tag performs communications. Accordingly, the operating characteristics of the RF tag change greatly between before and after its attachment to the conductive object.
Non-Patent Document 1 at the time of the filing of the present application carries a conventional RF tag attachable to metal.
[Non-Patent Document 1]    http://www.awid.com/webresources/documents/products/ MT%20Tag_LR-911%206-04.pdf
The conventional RF tag as described in Non-Patent Document 1 has an antenna structure so as to operate as a dipole antenna longer than a half-wavelength. More specifically, a conductive material representing an antenna pattern is provided on one side of a dielectric, and a metal layer is formed on the other side of the dielectric, so that the overall length is designed to be approximately a half-wavelength. Since the operating frequency is 902-928 MHz, the overall length is approximately 17 cm. However, there is a problem in that objects to which the RF tag is attached are greatly limited in type by this size.
Further, the antenna size and the material characteristics of the insulating layer of the conventional RF tag are determined so that it is possible to perform desired radio communications when the RF tag is attached to a conductive object. Accordingly, during the manufacturing process of the RF tag, it is difficult to use information in an integrated circuit in the RF tag through the antenna at a stage where only the conductive layer part of the antenna is prepared (an underlying dielectric layer and a ground conductive layer are not formed). Therefore, in the case of an RF tag to accompany a conductive object, unlike in the case of an RF tag to accompany a non-conductive object, there is a problem in that it is not possible to make effective use of information in the RF tag before its completion.